For instance, a gas sensor, which outputs an electromotive force, is provided at a vehicle engine (e.g., an automobile engine). In this type of gas sensor, exhaust gas, which is discharged from the engine, serves as a sensing subject of the gas sensor, and an oxygen concentration of the exhaust gas is sensed with the gas sensor. This type of gas sensor includes an electromotive force (EMF) cell, which outputs an electromotive force signal that varies depending on whether the exhaust gas is rich or lean. Specifically, when an air-to-fuel ratio is rich, the electromotive force cell outputs the electromotive force signal of about 0.9 V. In contrast, when the air-to-fuel ratio is lean, the electromotive force cell outputs the electromotive force signal of about 0 V.
In this type of gas sensor, when the air-to-fuel ratio of the exhaust gas changes between rich and lean, a change in the sensor output may be disadvantageously delayed relative to an actual change in the air-to-fuel ratio. In order to improve the output characteristic of such a gas sensor, various techniques have been proposed.
For instance, JP2012-063345A (corresponding to US2012/0043205A1) discloses a gas sensor control apparatus, in which a constant current circuit is connected to at least one of a pair of sensor electrodes (i.e., two sensor electrodes). In this gas sensor control apparatus, when it is determined that a demand for changing the output characteristic of the gas sensor is present, a flow direction of the constant electric current is determined based on the demand. Then, the constant current circuit is controlled to supply the constant electric current in the determined direction. Specifically, the constant current circuit can supply the constant electric current in any one of a forward direction and a backward direction and can adjust a current value of the electric current through a pulse width modulation (PWM) control operation.
However, in the above-describe technique, the supply of the constant electric current of the constant current circuit is controlled through the PWM control operation. In order to meet, for example, a cost reduction demand, an improvement may be made to simplify the structure.
In order to simplify the structure of the constant current circuit, the electromotive force cell, which is in the electromotive force generating state, is used as a battery, and a flow of an electric current may be induced in the electromotive force cell through use of the electromotive force generated by the electromotive force cell. However, in the case where the electromotive force of the electromotive force cell is used to induce the flow of the electric current in the electromotive force cell, the flow of the constant electric current, which is conducted through the electromotive force cell, is influenced by the voltage value of the electromotive force. In a state where the electromotive force of the electromotive force cell is relatively small, the current value of the constant electric current may possibly be excessively reduced to have an unexpectedly small value. That is, in the case of the electromotive force cell of the gas sensor, the voltage value of electromotive force cell changes in the range of 0 to 0.9 V. In the lean state, the current value of the constant electric current may possibly become the excessively small value, or the constant electric current cannot be conducted through the electromotive force cell. Therefore, there is a need for improvement.